Blade coaters are utilized extensively in the papermaking industry for applying coatings to paper webs directly on the papermaking machine as well as in off-line coating operations. Blade coaters are desirable for their simplicity of construction and relative ease of control. A blade bears against a paper web carried through an application zone on a backing roll, the blade tip being at an exit point of a fluid reservoir. Controllable pressure brought to bear against the tip of the blade controls the thickness of the liquid film applied to the paper web as it leaves the application zone.
In a blade coater when arranged in the operative position for coating a paper web, one end of the blade is fixed in the coater apparatus and the other end is free, bearing against the web which rides on the backing roller. Loading means, such as a single elongate pneumatic tube, is positioned to bear against the blade in a position relatively near the blade tip. Pressure in the pneumatic tube thereby controls the force of the blade against the roll (as well as the blade geometry) and thereby controls the liquid film thickness applied to the web. It has been found that such control is not adequate for all purposes. More particularly, it has been found that the tip loading applied to a singly loaded blade has a substantial effect on both tip geometry and tip loading as the control alters the pressure exerted against the blade. As a result, film thickness is often not controllable over an adequate range. More significantly, even within the controllable range, the responsiveness of such a control is often found to be inadequate, both in terms of the linearity of the control as well as the control slope, i.e., the increment of film thickness adjustment which can be obtained for an increment of the control variable. Thus, in a system where a typical pneumatic tube is used to load the flexible blade, and pressure within that pneumatic tube is the sole running controllable variable, it is sometimes found that the range of control is inadequate and even more frequently it is found that the fine adjustments which are sometimes desired are not often achievable. This result follows because a slight change in pressure in the pneumatic tube can alter both the blade tip geometry and the tip load, to cause a relatively substantial change in film thickness for a relatively minor change in the control variable.